I will assume that since the server came with hot-swap drive bays, they are already connected to a functional SCSI or Hardware RAID controller. The Windows NT 4.0 Operating System does not support dynamic updates of new (additional) disk hardware, so even if you install the new disk into an appropriate hot-swap slot, the server will have to be rebooted to recognize it.
If the hot-swap bays are connected with a simple SCSI controller, there shouldn’t be any extra hardware configuration necessary – just reboot the machine with the new drive installed, log into the server with an administrative account and use Disk Administrator (located in Administrative Tools) to configure the drive partitions the way you want.
If the hot-swap bays are connected to a Hardware RAID Controller you will need to use the Controller Manufacturer’s configuration utility, normally available as a boot-time option, to configure the drive before the operating system can recognize it. You will still need to finish configuring the drive with Disk Administrator after it’s recognized.
I am assuming you are using hardware raid. You will not be able to simply add the drive to the raid array. You will probably have to backup all data to tape, add the disk, use the manufacturers configuration software to format and reconfigure the raid array, reinstall the OS, and then restore the data to the new array.
The easier way is to install the drive as a standalone that is not part of the raid array.
Your best bet is to contact the manufacturer of your server for recommendations.